During the period of clinical latency HIV infected individuals exhibit a gradual reduction in CD4+ T cell numbers. The CD4+T cells which remain in these patients are hyporesponsive to antigen receptor stimulation. Therefore, it appears that HIV induced immunosuppression is the result of both decreased number and functional capability of CD4+ helper/inducer T lymphocytes. This application is focused on elucidation of the molecular basis of CD4+T cell hyporesponsiveness in HIV infected individuals. The HIV envelope glycoprotein (gp120) is responsible for the tropism of HIV for CD4+ T cells by virtue of its high affinity for CD4. In uninfected T cells, crosslinking of CD4 molecules by gp120-anti-gp120 complexes or HIV has been shown to suppress of T cell biologic function and uncouple T cell antigen receptor (TCR) signaling, a condition referred to here as desensitization. In peripheral T cells or in HPB-ALL cells, a human T cell line, gp120 induced desensitization is indicated by the failure of TCR ligation to trigger elevation of intracellular free calcium or induce protein tyrosine phosphorylation. These results suggest an uncoupling of the TCR from protein tyrosine kinases, the earliest effectors of TCR ligation known. Based on these findings, we hypothesize that CD4 T cell unresponsiveness seen in AIDS is mediated by the inactivation of an early intermediary in the TCR signaling pathway such as a tyrosine kinase or receptor transducer subunit. In this application, we propose to address this hypothesis by defining the molecular basis of gp120 induced TCR desensitization. In Aim I, we propose to define the site in the TCR mediated signal transduction cascade that is disrupted by gp120/anti-gp120 crosslinking of CD4. To assess the physiologic relevance of this uncoupling we will determine whether this condition is constitutive in CD4+ T cells from HIV-infected patients. Studies proposed in Aim 2 will determine the role of receptor subunit phosphorylation in mediating desensitization. Studies proposed in Aim 3 will determine if desensitization is mediated by sequestration or translocation of tyrosine kinases or phosphatases which modulate or regulate TCR signal transduction. Finally studies in Aim 4 will assess the role of CSK (a negative regulatory protein tyrosine kinase) activation and CD45 (a positive regulatory phosphotyrosine phosphatase) inactivation in mediating gp120 mediated TCR desensitization. These studies would provide important insight regarding the molecular basis of gp120?anti-gp120 mediated desensitization of T cell antigen receptor signaling and may lead to development of agents which, by blocking desensitization, are of therapeutic value in AIDS.